The present invention relates to color liquid crystal image displays using cholesteric liquid crystals.
The existing art for color cholesteric liquid crystal displays is to stack three individual color planes each on individual base substrates, as shown in FIG. 1. The problem with this is that the color planes must be accurately aligned, an air gap will exist which creates optical losses which must be addressed and when such a display is flexed the flexing radius of each base substrate is significantly different that pixel alignment will be effected. Also, the base material and conductive ITO layers attenuate the light reflecting off the display and reduce the brightness of the display. The conductors are patterned so as to provide an array of pixels. For an example of electronic circuitry for driving such an array, see U.S. Pat. No. 5,251,048.
U.S. Pat. No. 3,703,329 is similar to FIG. 1 and shows a stacked three color display, which does not use cholesteric liquid crystal. The three color display has three cells each including a solution with pleochroic dye in a nematic liquid crystal composition. Each of the solutions can change its transmission of polarized white light in response to an electric field so as to change the color appearance of the solution. One solution can be changed in appearance from color less to magenta, another from colorless to cyan, and a third from colorless to yellow. The display applies an electric field separately to each of the solutions and passes polarized white light successively through each solution such that when electric field is provided across the pixel, a different color pixel is presented to the viewer. This arrangement has problems in that high optical losses are associated with the multiple glass substrates which prevents producing high quality images and the alignment of the pixels is a problem because pixels in each of the three cells must be accurately aligned to one another in the assembly process.
It is therefore an object of the present invention to provide a color display, comprising:
a) a substrate defining first and second opposite support surfaces;
b) a first conductor disposed over the first surface and defining a first pattern;
c) a second conductor disposed over the second surface and defining a second pattern;
d) a first light modulating layer disposed over the first conductor including a liquid crystal material having a chiral dopant selected so that regions can be placed in a state where colored light is reflected in a first portion of the spectrum;
e) a second light modulating layer disposed over the second conductor including a liquid crystal material having a chiral dopant selected so that regions can be placed in a state where colored light is reflected in a second portion of the spectrum;
f) a third conductor disposed over the first light modulating layer and defining a third pattern;
g) a third light modulating layer disposed over the third conductor including a liquid crystal material having a chiral dopant selected so that regions can be placed in a state where colored light is reflected in a third portion of the spectrum;
h) a fourth conductor disposed over the third light modulating layer and defining a fourth pattern;
i) a fifth conductor disposed over the second light modulating layer and defining a fifth pattern; and
j) means for selectively applying voltages to different patterns of the first, second, third, fourth, and fifth conductors so fields are applied to selected regions of the first, second, and third light modulating layers to present a colored image to a viewer.
The present invention has an improved feature in that all three color planes are coated onto a single base substrate, FIG. 2. Additionally, two of the color planes share a common conductive ITO layer, thus eliminating one layer of ITO. The ITO patterning process can be reduced from six setups to just three.
A single base substrate display reduces the light attenuation from multiple base substrates and results in a brighter display. The air gaps between color planes is eliminated resulting in less optical losses and a brighter display. The elimination of an ITO layer reduces light attenuation as well as lowers costs. Finally, pixel alignment can be more accurately maintained when the display is flexed.